When a pot is heated on the top surface of a flat top electric heating range, it is often desirable to measure the temperature of the bottom surface of the heated pot. It is known that such heated surface emits infrared radiation. The present invention is directed toward apparatus for measuring such radiation and converting this measurement in to a corresponding temperature measurement.
The relationship between the temperature of a hot object as measured in degrees of temperature T, using the Kelvin temperature scale, and its spectral radiance L, when this relationship is measured using a wavelength of observation w, is defined by the well known Planck formula L[w]=[E][C1][w-5][exp{C2/wT}xe2x88x921-1] where E is the spectral emissivity of the hot surface and C1 and C2 are constants. Consequently, in order to determine the temperature of a hot object from its direct infrared radiation, it is necessary to know its emissivity as well as its radiance.
The infrared radiation emitted directly by the heated pot usually does not approach that of a black body at the temperature of the pot and its emissivity value does not approach that of a black body and hence has an emissivity value much less than E=1. However, as explained in more detail hereinafter, the radiation emitted directly by the heated pot can be enhanced so that the effective emissivity of the enhanced radiation approaches that of a black body. Moreover, the ratio of the enhanced radiance to the direct radiance is a unique function of the physical emissivity of the pot. Thus, applicant had determined that using the value of the direct radiation of the pot and the emissivity of the pot as computed from this ratio, the temperature of the heated bottom surface of the pot could be computed using the Planck formula.
However, applicant discovered that the electric range itself produced sufficient radiation to interfere with any measurement of the direct and enhanced radiation of the pot. In addition, unless great care was employed in making radiation measurements, the measurement equipment would be heated and produce self emission, which of course produced further interference.
This invention is directed toward a new type of measuring apparatus for measuring the temperature of such a heated pot in such manner that the radiation from the range could not interfere with the desired measurement and further that the apparatus could not be heated to a level at which significant self emission was produced. Moreover, this apparatus provides a means for determining the emissivity of the heated pot. Consequently, an accurate measurement of the temperature could be obtained using the Planck formula.
Apparatus in accordance with the principles of this invention is directed toward determining the temperature of a heated pot disposed over an opening in a top flat surface of a cooking range. For this purpose, the bottom surface of the pot is larger than this opening and covers it.
The apparatus employs an infrared transparent window disposed in the opening abutting said surface so that the window is flush with said surface.
An infrared reflective hemisphere is disposed below the window. The hemisphere has an open top surface in direct contact with the window and a closed bottom surface with a small opening therein. As a result, a direct infrared radiation component from the bottom of the pot and a reflected infrared radiation component from the exposed surface of the hemisphere both pass freely through the window and the mixture of these two components is essentially combined within the cavity formed by the pot and hemisphere, enhancing the direct radiation from the pot so that the combined mixture approaches that of a black body at the temperature of the pot.
The apparatus employs a first infrared wave guide coupled to the small opening in the hemisphere to receive the enhanced radiance from the pot and a second wave guide butted against the window to receive the direct pot radiance.
The apparatus also employs first means disposed below said small hemisphere opening and coupled to said first and second guides for deriving from the guides the values of the direct radiance and the enhanced radiance; and second calculating means coupled to said first means. The second means has stored therein a program for determining the emissivity of the pot from the ratio of the enhanced radiance to the direct radiance, and incorporating the direct radiance value and the emissivity in the Planck formula to compute the temperature of the pot.
In order to prevent the radiation from the cooking range from interfering with the temperature measurement, applicant utilizes in the first means infrared radiation detectors which do not respond to the wavelengths of radiation from the range but respond only to a different and non-overlapping group of wavelengths whereby range radiation cannot interfere with temperature measurements. Moreover, the apparatus incorporates additional means to prevent it from being heated to such a level that self emission can interfere with the temperature measurements.
As a result, this invention overcomes the prior art difficulties in measuring the temperatures of heated pots employing flat top cooking ranges and for the first time enables accurate temperature measurements to be obtained.